Electric heater and method of making the same



Nov. 12, 1929. E. N. LIGHTFOOT ELECTRIC HEATER AND METHOD OF MAKING THE SAME Original Filed May 7, 1925 Patented Nov. 12, 1929 UNITED STATES PATENT OFFICE EDWIN N. LIGHTFOOT, OF WAUWATOSA, WISCONSIN, AS|'5IGINOIB,, BY um ASSIGN- MENTS, TO CUTLER-HAMMER, INC., A CORPORATION OF DELAWARE ELECTRIC HEATER AND METHOD OI MAKING THE sum Application filed Kay 7, 1925, Serial No. 28,558. Renewed April 90, 1929 This invention relates to electric heaters and methods of making the same, and, more particularly, to heaters of the cartridge type.

Cartridge heaters, so called because of design thereof to be slipped into drilled recesses in the parts to be heated, preferably comprise a resistor wound on a core, a cylindrical metallic enclosing casing and insulation interposed between the resistor and its casing. Also, such a heater preferably has both of its terminals arranged at one end thereof, and to facilitate conduction of heat from the resistor to the casing, it is desirable to reduce the interposed layer of insulation to minimum thickness. Accordingly, it will be apparent that satisfactory construction of such a heater presents diflicult problems as regards insulation of the current carrying parts, accommodation of the through lead wire, anchorage of the lead wires within the heater, etc., especially where the heater is of small diameter.

It has been proposed to employ in such heaters powdered or granular magnesium oxide to insulate the electrical parts and to compact such material by swaging the casing for better heat conduction, this type of insulation having recognized advantages. However, since the cartridge type heater is closed at its ends and comprises an insulating core, and since the resistor usually comprises a fine wire with closely wound turns, the swaging process has obvious disadvantages as applied to this type of heater.

The present invention has among its objects to obviate, or, at least minimize, the aforementioned and other difficulties heretofore experienced in the production of cartridge heaters.

Another object is to provide an extremely satisfactor heater of the cartridge type utilizing pow ered or granular magnesium oxide or the like without need of swaging.

Another object is to provide a cartridge heater wherein the lead wires are firmly anchored.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawing illustrates a Fig. 2 is a transverse sectional view taken on line 22 of Fig. 1';

Fig. 3 is a fragmentary view of the heatin e ement; and

ig. 4 is an end view of said heating element.

As illustrated in Fig. 1, the heater com-' prises a resistor 5 wound on an insulating core 6 and enclosed in a cylindrical casing 7, preferably metallic, and containing insulatin material 8 to support the resistor in a space relation with respect to said casing.

The core 6 supporting the resistor may comprise any suitable material and may be either solid or hollow. Preferably, however, the core is in the form of a tube as shown, and, preferably, it comprises a winding of sheet asbestos the layers thereof being glued together with a binder such as silicate of soda makingia comparatively thin stifi' wall tubing su ciently strong to carry the resistor. This form of core is particularly adapted to heaters of small diameter, although suitable for heaters of larger diameter.

.The resistor comprises any suitable wire wound on the core and preferably covered by a thin coating of cement for preventing shifting of the turns thereof during subsequent steps of manufacture, although it is to be understood that use of this cement is optional. The resistor being wound as shown has its terminals at opposite end thereof, and said terminals are respectively provided with lead wires 9 and 10, both projecting from the same end of the casing. These lead wires may be of any preferre po and may be connected to the terminals of the resistor wire in an preferred manner.

Preferably the lead wires comprise asbestos insulated flexible wires, and preferably such wires are connected to the terminals of 4. The core is provided at opposite ends with notches 11 such as that shown in Fig. 3 to receive the bared ends of the lead wires which are placed in these notches when the core is placed on the winding mandrel, said wires aving the strands thereof separated to lie flush with the surface of the core. Thus, when the resistor is wound on the core it will have a number of end turns in contact with the end of the lead wires, ermanent connection being made between t e resistor and the lead wires by silver solder or in any other preferred manner. As shown, the lead wires are located within the core, one extending the full length thereof and the other being reflexed for a pu ose hereinafter set forth.

- The casing 7 is provided at one end with a bushing 12 formed of lava or other suitable material and having an annular shoulder 13 .over which the end of the casing is s un for support of'said bushing. Also, the ushing 12 has a reduced portion 14 on which the core of the resistor is adapted to be mounted for support thereof in a centered relation with the casing. Further, the bushing 12 is provided with passages 15 and 16 through which the lead wires extend.

At its opposite end the casing is constructed to receive a closing disk 17, said casing having a shoulder 18 constituting an abutment for said disk. As will be understood the disk 17 is inserted in the casing only after assembly of the elements aforedescribed, securement of said disk being effected by spinning thereover the end of the casing. Although perhaps not essential an insulating disk 20 is preferably interposed'between the disk 17 and the resistor to obviate possibility of grounding of the resistor on the disk 17 which ordinarily would be of metal.

The insulating material 8 preferably comprises granular fused magnesium oxide which is introduced into the casing and into the bore of the core just prior to closing of the end of the casing by disk 17. The magnesium oxide is preferably packed by vibrating the same when introduced into the casing for the purpose of reducing the voids therein and for aflording firm support for the free end of the core member during the treatment of the insulating fillin now to be described.

When the heater 1s thus filled, the magnesium oxide contained therein is subjected to chemical reaction to convert the same into a hydroxide form for further disintegration of its particles-with a consequent expansive action which causes the particles to distribute and to fill all appreciable voids within the casing which preferably is designed or confined to withstand the pressures thus produced therein, The magnesium oxide when so treated is then reconverted into its original chemical state whereupon it is rendered satisfactory both as an electric insulator and heat conductor, and, as will be apparent, the

disadvantages of swaging are thus obviated. Also, the aforementioned treatment of the ma esium oxide causes the same to grip the lea wires throughout the embedded len hs thereof, thereby firmly anchoring the ead wires within the heater and preventing strains on the connections between the same and the resistor.

The magnesium oxide filling of the casing maybe readily converted as aforedescribed by introducin into the casing a quantity of water to fill al voids and then subjecting the filling to the action of heat and pressure. A convenient method of thus treating the filling is to lace the heater in an autoclave subjecting t e filling to the action of steam in the neighborhood of 330 F. or more for a suitable period of time. This treatment will effect the aforementioned disintegration of the magnesium oxide and thereafter the same may be restored to its original chemical state by subjecting it to a high temperature, preferably red heat, to drive off all moisture. While such reconversion of the material effects a reduction in its weight, the apparent volume of the material is not changed by the reconversion, thus preserving its aforementioned advantages gained by its conversion into a hydroxide state.

It is, ,of course, to be understood that other insulating materials adapted to the aforementioned treatmentmay be employed in lieu of magnesium oxide, and that the effects of the reaction may be controlled in various ways, as, for exam le, by mixing with the active material a sultable quantity of chemically inert material or by controlling the progress of the reaction.

What I claim as new and desire to secure by Letters Patent is:

1. A heater comprising an insulating core, a resistor wound thereon, a casing enclosing said resistor and insulationbetween said resistor and casing chemically disintegrated in situ.

2. A heater comprising a hollow insulating core, a resistor wound 'thereon, a casing enclosing said resistor and insulation within said core and between said resistor and casing chemically disintegrated in situ.

3. In combination a wound resistor, a core supporting the same, a casing enclosing the aforementioned elements and an insulating filling for said casing surrounding said resistor and core, said filling comprlsing particles multiplied in number by chemical reaction of such filling in situ with consequent expansive action opposed by said casing.

4. In combination a wound resistor, a hollow core supporting the same, a casing enclosing the aforementioned elements and insulation within said core and surrounding said core and resistor, said insulation comprising particles multiplied in number by chemical reaction of such insulation in situ with consequent expansive action opposed by said casin 5. A heater comprising a resistor, lead wires connected thereto, a hollow element associated with said resistor and in which portions of said lead wires are located, andinsulation within said hollow member chemically disintegrated in situ to produce a mass anchor ing said lead wires in said hollow member.

6. A heater comprising a wound resistor, a hollow core sup ortingthe same, lead wires for said resistor avin portions located within said core and ins ation within said core chemically treated in situ with expansive tendencies to produce a mass anchoring said lead wires in said hollow member.

7 A heater comprising a wound resistor, a hollow core supportin the same, lead wires for said resistor, one 0 said wires extending through the bore of said core and-the other having a reflexed portion within the core, a casin surrounding said resistor and an insulating filling for said casing and core chemically treated in situ with expansive tendencies to produce masses anchoring said lead wires in said core and anchoring said core in said casing.

8. A heater comprising a tubular casing having at one end an insulating bushing, an

insulating core engaged at one end with said I bushing to be centered in said casing, a resistor wound on said core andhaving lead wires extending through said bushing and an insulating filling for said casing chemically treated in situ with expansive tendencies opposed by said casing.

9. A heater comprising a tubular casing having at one end an insulating bushing, a

hollow insulating core having one "and engaged with said bushing for centeringlsaid core in said casin a resistor wound on'said ,atu

to lead wires respectively located within said core, attaching said core at one end to an insulating bushing for centering thereby with said lead wires'penetratin'g the latter, insert ing said bushin and the parts carried thereby within a meta lic casing, filling the space within said core and around said core and resistor with granular fused magnesium oxide, and then subjecting said magnesium oxide to hydration and dehydration treatment in situ to provide masses anchoring said lead wires in said core and anchoring said core in said casing.

12. The method of making industrially useful unswaged electric heaters of the cartridge type which consists in forming a thin walled insulating tube of sheet asbestos, winding a resistor upon said hollow tube, connecting the opposite ends of said resistor to lead wires located within said tube, one 7 of said lead wires extending throughout the length of said tube and the other lead wire being reflexed adjacent one end of said tube, attaching said tube at one end to an insulating bushing through which the lead wires are adapted to pass placing the parts thus assembled within bular metallic casing with a portion of said bushing projecting beyond one end of said casing, inserting a quantity of granular fused magnesium oxide through the other end of said casing to fill the s ace within said tube and around said tube an resistor, permanently closing said last mentioned end of said casing, and then subjecting said magnesium oxide to hydration and dehydration treatment in situ to provide coherent masses anchoring said lead wires in said tube and anchoring said tube in said casing.

In witness whereof, I have hereunto subscribed my name. 1 EDWIN N. LIGHTFOOT.

core, lead wires or said resistor extending through said bushin one of said lead wires extending through t e bore of said core to the remote terminal of said resistor and the other having a reflexed portion disposed within said core and insulationwithin said core and surrounding said core and resistor, said insulation being chemically treated in situ with expansive tendencies opposed by said casing.

10. The methodof making electric heaters which consists in mounting a helical resistor upon an insulatin core, lacing said core in a metallic casing, fi lin tli e space around said core and resistor wit ranular heat refractory insulating materia and then chemically treating said insulating material in situ to efiect conversion of the same into a coherent mass without change in the apparent volume 

